In recent years, there has been vigorous development of high-breakdown voltage, high-output compound semiconductor devices, making use of advantages of nitride-based compound semiconductor including high saturation electron mobility and wide band gap. The development is directed to field effect transistors such as high electron mobility transistors (HEMTs), for example. Among them, a GaN-based HEMT having an AlGaN layer as an electron supply layer attracts a lot of attention. In the GaN-based HEMT, lattice distortion occurs in the AlGaN layer due to difference in lattice constants between AlGaN and GaN, the distortion induces piezo polarization therealong, and thereby generates a high-density, two-dimensional electron gas, in the upper portion of the GaN layer laid under the AlGaN layer. This configuration ensures high output.
However, it is difficult to obtain normally-off transistors due to high density of the two-dimensional electron gas. Investigations into various techniques have therefore been directed to solve the problem. Conventional proposals include a technique of disconnecting the two-dimensional electron gas by etching a portion of the electron supply layer just below the gate electrode, and a technique of vanishing the two-dimensional electron gas by forming a p-type GaN layer containing Mg as an acceptor impurity between the gate electrode and the electron supply layer.
Etching of the portion of the electron supply layer just below the gate electrode will, however, damage the electron channel layer, to thereby induce problems of increase in sheet resistance and increase in leakage current. On the other hand, formation of the p-type GaN layer containing Mg will elevate resistivity. In this way, conventional efforts to obtain the normally-off transistors have degraded other characteristics of the transistors.
[Patent Literature 1] Japanese Laid-Open Patent Publication No. 2009-076845
[Patent Literature 2] Japanese Laid-Open Patent Publication No. 2007-019309